1. Field of the Invention
The present invention relates to a method for producing a coplanar waveguide system on a substrate for the transmission of electromagnetic waves and a component fabricated in accordance with such a method.
2. Description of the Background Art
With increasing operating frequency, component modeling of components integrated on a semiconductor substrate is playing an increasingly bigger role because it causes transmission-line characteristics, reflections on discontinuities, overlapping and dissipation to increase. That makes it generally imperative to consider these effects in the modeling process, particularly in the high frequency field. Particularly with a low-resistance substrate, for example, a silicon substrate, the parasitic influence of the substrate conductivity and additional capacitance must not be neglected.
Although generally applicable to any circuit line or any passive component, the present invention and the problems it is based on are described in detail with regard to a coplanar waveguide (CPW).
Since technology in the radio frequency field is shifting from big systems with a wide transmission range to smaller systems with a more limited range, and more and more newer systems are mobile ones, the trend in the RF field is to build radio-frequency-suitable apparatuses that are more economical and easier to use. In recent years, so-called coplanar wave guides, which have considerable advantages over the conventional micro-strip technology, have therefore been explored with increasing frequency. For example, dispersion due to power transfer by air is lower with a coplanar waveguide system, and parasitic interferences, for example, discontinuities, are lower than with conventional micro-strip devices. Furthermore, no through holes are required, so that the mechanically non-stable semiconductors do not have to be of such an extremely thin construction.
The coplanar wave guide is a planar three-line system, generally comprised of a signal conductor and two grounding conductors that are symmetrically arranged thereto. The coplanar wave guide, in correspondence with the three conductors, has two fundamental waves that are commonly referred to as coplanar mode and slot line mode. From a technical viewpoint, however, only the coplanar mode is desired, therefore, air bridges always have to be in place to prevent the second mode from spreading.
According to conventional technology, such a coplanar waveguide generally includes three metal strips, which extend parallel to one another and are embedded in a silicon oxide layer, for example. The oxide layer between the metallization and the low-resistance carrier substrate must thereby be as thick as possible in order to keep the substrate losses as low as possible.
The disadvantage of this conventional approach, however, has proven to be the fact that by direct coupling of the coplanar wave-guide system, that is, the individual conductors of the coplanar wave guide with the dielectric layer, that is, the substrate, high line transmission losses, high substrate losses and minimal muting of the interactions of the individual modes with each other occur. Thus, undesired effects like emission, cross coupling of signals, or oscillations of amplifier circuits etc. occur, particularly in the high frequency field.
It is therefore generally desirable to keep the conductor losses of a coplanar waveguide system as low as possible. In a conventional approach, a micro-screened line system was constructed, whereby a middle of the signal conductor and the grounding conductors arranged parallel thereto are at least partially surrounded by air, whereby the individual conductors are supported by a 1.5 μm-thick membrane, for example, whereby an air gap is provided below the membrane. Thus, a single mode, that is, wave propagation over a very wide band range with reduced dispersion and a reduced dielectric loss can be achieved. With a metallized shielding cavity below the line system, couplings between neighboring lines and interference modes in the substrate are reduced.
The disadvantage of this conventional approach, however, has proven to be the fact that the conventional fabrication of a micro-screened coplanar wave guide depends on the technology for the fabrication of the thin dielectric membrane and also on the anisotropic etching process of the carrier substrate. The conventionally used membrane is composed of a three-layer construction of SiO2—Si3N4—SiO2. The production method of such a three-layer-construction is costly and complicated and requires at least two steps. To start with, an opening in the silicon nitrate layer on the back side of the substrate is defined and subsequently, the substrate is back-etched until a transparent membrane evolves. Next, various geometries suitable for micro-screening are formed by using photolithography. Thus, this production method is labor-intensive and costly, whereby the metallizations can only be made relatively thin resulting in high line transmission losses and high electrical resistance values.
In addition, this conventional approach has the disadvantage that the upper grounding points and the lower mass conductors are not directly interconnected but are separated from one another by a dielectric layer. Thus, the individual grounding points have to be grounded separately from one another, which requires additional expenditure in labor.